Since the discovery by Chu and coworkers of the compound YBa.sub.2 Cu.sub.3 O.sub.7 and the demonstration that it undergoes a transition to the superconducting state above 90K, many groups around the world have tried to develop methods to grow single crystals of YBa.sub.2 Cu.sub.3 O.sub.7. Progress in the commercial development of superconducting electronic devices utilizing the superconductor YBa.sub.2 Cu.sub.3 O.sub.7 (also referred to in the literature as "123") requires the availability of high-quality single crystals.
Previous reports which have appeared in the scientific literature disclose attempts to grow single crystals of YBa.sub.2 Cu.sub.3 O.sub.7 which produced non-superconducting or weakly superconducting crystals as-grown, with a superconducting transition temperature (T.sub.c) below 70K. Transition widths of these crystals as-grown are very broad (10-20K). For a compendium, see the special issue on high-Tc superconductors: Journal of Crystal Growth 85, (4) 563-677 (1987). Also, see Dinger et al., Phys. Rev. Lett. 58, 2687 (1987); Worthington et al., Phys. Rev. Lett. 59, 1160 (1987); Kaiser et al., Appl. Phys. Lett. 51, 1040 (1987); Schneemeyer et. al., Nature 328, 601 (1987); Hikata et al., Phys. Rev. B 36, 7199 (1987).
The previously reported attempts to grow single crystals of superconducting YBa.sub.2 Cu.sub.3 O.sub.7 entail a flux mixture of Ba0, Cu0 and Y.sub.2 O.sub.3. The as-grown crystals obtained by these reported processes were not superconducting above 90K. The poor superconducting properties of these crystals as-grown reflect a serious oxygen deficiency in the crystals incurred by the wrong growth conditions. A subsequent very long oxygen annealing was required to bring the oxygen content of the crystals up to the desired 7/formula unit. (See Kaiser et al., Jnl. Crystal Growth 85, 593, 1987.) Kaiser et al. disclosed that their as-grown crystals have a broad transition with -a mid-point at 60K. Only after oxygen annealing these as-grown crystals for ten days at 420.degree. C. was the transition temperature improved to 92K and the transition became sharp. Similar results have been reported by Schneemeyer et al., Nature, 328, 601 (1987). Other authors report that the crystals as-grown according to their methods are tetragonal, that is, the oxygen content is less than 6.5 per formula unit. (See Jnl. Crystal Growth 85 (4), 1987)